This invention relates to coating methods, products used in coating methods, and articles coated by such methods and products. In a particular aspect the invention is concerned with methods for the abradable coating of turbine engine shroud segments, products for forming coatings thereon, and coated articles, particularly turbine engine shroud segments with abradable coatings. More specifically, the invention relates to improved methods and products for effecting such coatings, and better coated articles. The invention improved coating products include flexibly formable biscuits for coating turbine engine shroud segments, which are laminar in form to provide different thicknesses of coating material for different applications, from shrouds, to turbine blade leading edges, to blade tips, and to other applications. The biscuits of the invention will conform generally to the curvature of any part. The invention biscuits are readily produced from mixtures of alloy powders of wider compositional variety than previously known biscuits for the purpose, and contain stripping agents or stripping agent precursors effective to strip oxides, such as aluminum oxide, or yttrium oxides from the substrate surface and from the coating composition during formation of the coating and thereby ensure maximum efficiency in bonding development between the substrate and coating.
In another aspect, the invention provides novel brazing compositions and methods for fabrication of the coating biscuit products. In this aspect the invention utilizes diffusion alloy processes, such as cementitious pack diffusions, to controllably add, in optimum distribution, fusion temperature lowering amounts of silicon to MCrAl and MCrAlY alloy powders to be subsequently converted into biscuits and formed into coatings on metal parts. In a further aspect, the invention provides methods of fabricating MCrAl, MCrAlY and other alloy powders into useful forms, suitably laminar and separable into layers of different thicknesses for different depths of coating on a selected metal part by the use of resinous processing aids such as polytetrafluorethylene.
In the aspect of the invention concerned with production of coated turbine engine shrouds. The old technology improved upon by the present invention is set forth in U.S. Pat. Nos. 4,842,953 and 4,937,042, both to Perkins and Smith, referred to herein as the Perkins patents. In these patents the patentees describe the need in gas turbine engines to achieve as great a reduction in leakage as possible between stationary and rotating gas turbine members and the use of abradable seals on turbine engine shroud segments to effect control of leakage. These patents describe the formation of suitable abradable seals on turbine engine shrouds by a biscuit process in which two specific, composition-limited powder alloys are mixed and compacted to a preform exhibiting self-supporting xe2x80x9cgreen strengthxe2x80x9d. The preform is suitably pre-sintered by taking its temperature to a temperature at which the lower, but not the higher melting alloy melts, i.e. above the liquidus temperature of the lower melting alloy but not as high as the liquidus temperature of the higher melting alloy. The resulting preform is a xe2x80x9cbiscuitxe2x80x9d of fixed shape; thus fixed in shape the biscuit must of necessity be closely congruent with the article to be coated, e.g. a shroud surface, since the preform biscuit by the Perkins patent process is brittle and cannot subsequently be flexed to conform to any surface but the intended one. Once securely placed on the shroud surface, the biscuit preform is heated to a temperature at which the lower melting alloy powder liquid phase sinters the powder composition, and, then further heated to temperatures at which the interdiffused powered alloys fuse together and bond to the substrate. The patentees require defined levels of silicon and yttrium in the overall composition, the presence of one and not the other in the respective alloy powders, which with other criteria imposed severely limits the number of alloys useful in the method and the variety of products to be obtained. Alloys containing substantial amounts of aluminum, yttrium, or titanium, or other metals prone to form surface oxides, such as Inco 738 containing about 4% by weight of each of aluminum and titanium, are advantageously coated with the compositions and methods of this invention. By the use of the invention compositions and methods such oxides are stripped from the coating composition and from the surface of the alloy articles being coated, as well, allowing bonding where the presence of these oxides had been thought to preclude it. For yttrium containing alloys such as CO-159, increased amounts of yttrium can be used in coatings in accordance with the invention beyond the 0.1% by weight taught as the maximum in the Perkins patents. This is because, in contradistinction to the Perkins patents, the present invention provides for stripping the oxides generated in the coatings or on the article surface before coating. Oxide presence is inimical to effective bonding of the biscuit to the substrate.
It is an object of the present invention to provide an improved method of coating turbine engine shrouds. It is another object to provide such a coating method using biscuits which are flexible to be conformed to virtually any shape of substrate surface. It is a further object to provide a method of making biscuits for such coating methods which is not limited in which alloys it can use by narrow constraints on the presence and quantities of silicon and/or yttrium, aluminum or titanium and novel biscuits by such method. Yet another object is to provide biscuit compositions which are self-stripping of oxides, and coatings and coated articles made therefrom which are stripped of oxides in the course of coating formation as a result of the biscuit composition. Still another object is to provide a repair composition, suitably but not necessarily in the form of a biscuit, for repair of metal parts, including gas turbine components such as shrouds, blade surfaces and blade tips, and components of all types, e.g. where an abradable surface coating is desirable. Another object is to provide improvements in the processing of superalloy powders into coating products such as biscuits. Other objects include provision of coating preparation methods which enable the use of a wide number of superalloys, and not just those having ab initio the desired amount of silicon present for fusion, and fabrication methods which simplify biscuit formation, provide flexible biscuits, and as well incorporate oxide-stripping components in situ in the coating composition.
These and other objects of the invention to become apparent hereinafter are realized by provision of a novel biscuit of a widely variable composition, which, when coated onto a shroud or other substrate, forms an effective coating, such as an abradable coating. More particularly, the invention objects are realized by modifying in a cementitious diffusion pack any of numerous alloys to incorporate silicon into the alloys without change in the basic composition. These diffusion-modified alloys are then used to distribute silicon highly uniformly through the biscuit and thus through the coating composition which comprises the diffusion-modified alloy, now a lower melting alloy, and a second, higher melting alloy; their combination forming a braze coating composition in which the lower melting alloy is the low temperature fusing, sintering alloy in the blend with a higher melting suitable alloy.
In another aspect of the invention, these objects are further realized by adding a source of hydrofluoric acid to the alloy blend to act as an oxide stripper upon final melt formation of the abradable coating or repair. The HF source is suitably in the form of a high molecular weight fluorinated polymer which will facilitate biscuit formation and biscuit flexibility before volatilizing as HF and other products in the final steps of the method. The use of the noted polymers further facilitates wetting of the composition powders by the silicon-containing lower melting alloy, enabling easier processing through the use of the polymer processing aid and improved fusion response in the composition. The improved wetting property in the polymer-containing powdered alloy blends of the invention removes a serious limitation on the presence of yttrium in the alloys. Yttrium content, e.g. in the previously mentioned patents, to less than 0.1% by weight, has been limited so as to avoid difficulties in wetting of the powders during fusion. This limiting of yttrium content is both unfortunate and unnecessary as shown by the present method where effective and efficient wetting of the powdered alloy blend is obtained with as much as 0.5% by weight of yttrium, i.e. five times as much, and even higher amounts of yttrium up to 1% by weight and more, in the powdered alloys when there is also present the polytetrafluoroethylene polymer.
The invention, accordingly, includes the method of making an article having an environmentally resistant, abradable surface, including, for particular compositions:
(a) Providing a powder mixture of first and second powdered alloys each having a base of elements M, Cr and Al in which M is at least one of the group of Ni, Co and Fe, the mixture having a total mixture composition of, by weight, 10-35% Cr, 4-19% Al, a small but effective amount of Y, 2-6% Si and the balance M, and further characterized by a substantial absence of B,
the first of the powdered alloys having a solidus and a liquidus, constituting from about 30 to about 50% by weight of the mixture, and comprising by weight 1.5-4% Al, 10-30% Cr and the balance M,
the first alloy characterized by having silicon diffused thereinto in melting range lowering amount between about 5% and about 18% by weight,
the second of the powdered alloys having a solidus and a liquidus, constituting from about 50 to about 70% by weight of the mixture, and having a melting range higher than the first of the powdered alloys,
the second alloy characterized by a substantial absence of Si,
the first and/or second alloy characterized by the presence of Y in an amount, up to a total of about 1.0% by weight, which will provide the Y content in the total mixture composition;
(b) Compacting the mixture at compacting temperatures below the solidus of the first of the powdered alloys to provide a preform;
(c) Disposing the preform on a surface of the article;
(d) Heating the preform and article surface in a nonoxidizing atmosphere above the liquidus temperature of the first powdered alloy and below the liquidus of the second powdered alloy of the mixture; and
(e) Holding the preform and article surface at the processing temperature for a time sufficient to liquid phase sinter the powder mixture, to interdiffuse elements of the first and second powders, and bond the preform to the article surface.
In this method, there is typically further included dispersing an HF precursor in an oxide purging amount in the mixture before heating at processing temperature to generate HF, selecting polytetrafluoroethylene as the HF precursor, combining the first and second powdered alloys with an alloy powder binding amount of polytetrafluoroethylene and compacting to provide the preform, and, heating the first alloy together with silicon in a diffusion pack to diffuse silicon into the alloy powder in advance of adding the silicon diffused first powdered alloy to the mixture.
In another embodiment of the invention there is provided a method of making an article having an environmentally resistant, abradable surface, including:
(a) Providing a powder mixture of first and second powdered alloys each having a base of elements M, Cr and Al in which M is at least one of the group of Ni, Co and Fe, the mixture having a total mixture composition of, by weight, 10-35% Cr, 4-19% Al, a small but effective amount of Y, 2-6% Si and the balance M, and further characterized by a substantial absence of B,
the mixture being bound together with a binding amount of polytetrafluoroethylene,
the first of the powdered alloys having a solidus and a liquidus, constituting from about 30 to about 50% by weight of the mixture, and comprising by weight 8-12% Si, 1.5-4% Al, 10-30% Cr and the balance M,
the second of the powdered alloys having a solidus and a liquidus, constituting from about 50 to about 70% by weight of the mixture, and having a melting range higher than the first of the powdered alloys,
the second alloy characterized by a substantial absence of Si, the first and/or second alloy characterized by the presence of Y in an amount, up to a total of about 1.0% by weight, which will provide the Y content in the total mixture composition;
(b) Compacting the polytetrafluoroethylene-bound mixture at compacting temperatures below the solidus of the first of the powdered alloys to provide a preform;
(c) Disposing the preform on a surface of the article;
(d) Heating the preform and article surface in a nonoxidizing atmosphere above the liquidus temperature of the first powdered alloy and below the liquidus of the second powdered alloy of the mixture; and
(e) Holding the preform and article surface at the processing temperature for a time sufficient to liquid phase sinter the powder mixture, to interdiffuse elements of the first and second powders, to volatilize off HF from the polytetrafluoroethylene, and to bond the preform to the article surface.
In this and like embodiments the invention the method typically includes compacting the polytetrafluoroethylene-bound mixture between protective covers with repeated passes through a pair of rolls, and using a processing temperature between about 1200xc2x0 F. and 2400xc2x0 F. in step (e) of the method.
In another aspect of the invention there is provided a method of repairing an article including the steps of:
(a) Preparing at least a portion of the article to provide a repair receiving surface;
(b) Selecting from the powder mixture of first and second powdered alloys each having a base of elements M, Cr and Al in which M is at least one of the group of Ni, Co and Fe, the mixture having a total mixture composition of, by weight, 10-35% Cr, 4-19% Al, a small but effective amount of Y, 2-6% Si and the balance M,
the first of the powdered alloys having a solidus and a liquidus, constituting from about 30 to about 50% by weight of the mixture, and comprising by weight 1.5-4% Al, 10-30% Cr and the balance M,
the first alloy characterized by having silicon diffused thereinto in melting range lowering amount between about 5% and about 18% by weight,
the second of the powdered alloys having a solidus and a liquidus, constituting from about 50 to about 70% by weight of the mixture, and having a melting range higher than the first of the powdered alloys,
the second alloy characterized by a substantial absence of Si,
the first and/or second alloy characterized by the presence of Y in an amount, up to a total of about 1.0% by weight, which will provide the Y content in the total mixture composition, as a replacement powder mixture which will provide a replacement article portion having a solidus above the incipient melting temperature of the article;
(c) Disposing the replacement powder mixture on the repair receiving surface;
(d) Heating the replacement mixture and repair receiving surface in a non-oxidizing atmosphere at a processing temperature above the liquidus temperature of the second powdered alloy and below the liquidus temperature of the first powdered alloy of the mixture; and
(e) Holding the replacement mixture and repair receiving surface at the processing temperature for a time sufficient to liquid phase sinter the powder mixture, to interdiffuse elements of the first and second powders, and to bond the replacement mixture to the repair receiving surface.
In this and like embodiments, typically, the method also includes binding the mixture with a binding amount of polytetrafluoroethylene before disposing on the article surface.
In a further embodiment of the invention, there is provided a method of making an article having an inner portion of a superalloy selected from a group consisting of Co-base, Ni-base, or Fe-base superalloys and having an incipient melting temperature, and an environmentally resistant, abradable outer portion metallurgically bonded with the inner portion, the outer portion consolidated and liquid phase sintered from a powder mixture of two powdered alloys each comprising a base of elements M, Cr and Al in which M is at least one element selected from a group consisting of Co and Ni, the mixture having a total mixture composition consisting essentially of, by weight, 10-35% Cr, 4-10% Al, from a small but effective amount of Y, 2-6% Si with the balance M,
the first of the powdered alloys comprising 30-50% by weight of the mixture, characterized by being diffused with silicon, and comprising, by weight, 8-12% Si, 1.5-4% Al, 10-30% Cr, with the balance M,
the second of the powdered alloys constituting 50-70% by weight of the powdered mixture and characterized by the substantial absence of Si, by having a solidus and a liquidus, and by having a melting range higher than the first of the powdered alloys,
the first and/or second powdered alloys characterized by the presence of Y in an amount, up to a total of about 1.0% by weight, which will provide the Y content in the total mixture composition,
including the steps of:
(a) Compacting the powder mixture of the outer portion at a compacting temperature below the solidus of the second of the powdered alloys to provide a preform;
(b) Disposing the preform on a surface of the inner portion;
(c) Heating the preform and surface in a non-oxidizing atmosphere at a processing temperature above the liquidus temperature of the first powdered alloy and below the liquidus temperature of the second powdered alloy of the mixture; and
(d) Holding the preform and the surface at the processing temperature for a time sufficient to liquid phase sinter the powder mixture, to interdiffuse elements of the first and second powders, and to bond the preform to the surface of the inner portion.
In this and like embodiments, typically, the method includes selecting a turbine shroud as the article, and the invention includes a turbine shroud made by this method and by the method which further includes binding the first and second powdered alloys with polytetrafluoroethylene in advance of the compacting step, and volatilizing HF from the polytetrafluoroethylene in the interdiffusion of elements step in oxide stripping relation.
The invention provides in another aspect a composition comprising a mixture of two powdered alloys each comprising a base of elements M, Cr and Al in which M is at least one element selected from the group consisting of Ni, Co and Fe, the mixture having a total mixture composition comprising by weight, 10-35% Cr, 4-10% Al, from a small but effective amount of Y, 2-6% Si, with the balance M, the Si being present solely as a diffused element into the lower temperature range melting one of the powdered alloys.
In this connection there is provided a more particular composition comprising a mixture of two powdered alloys each comprising a base of elements M, Cr and Al in which M is at least one element selected from the group consisting of Ni, Co and Fe, the mixture having a total mixture composition comprising by weight, 10-35% Cr, 4-10% Al, from a small but effective amount up to about 0.09% Y, 2-6% Si, with the balance M, the Si being present solely as a diffused element into the first of the powdered alloys, and further characterized by the substantial absence of B;
the first of the powdered alloys having a solidus and a liquidus, constituting 30-50% by weight of the mixture, being characterized by the substantial absence of Y, and comprising, by weight, 8-12% Si, 1.5-4% Al, 10-30% Cr, with the balance M,
the second of the powdered alloys having a solidus and a liquidus, constituting 50-70% by weight of the mixture and having a melting range higher than the first of the powdered alloys;
the second alloy characterized by the substantial absence of Si,
the second powdered alloy characterized by the presence of Y in an amount, up to a total of about 0.15% by weight, which will provide the Y content in the total mixture composition.
The invention further provides an article including an environmentally resistant, abradable surface portion comprising:
an inner portion of a superalloy selected from a group consisting of Co-base, Ni-base or Fe-base superalloys and having an incipient melting temperature; and
an outer portion metallurgically bonded with the inner portion, and consolidated and liquid phase sintered from a mixture of two powdered alloys each comprising a base of elements M, Cr and Al in which M is at least one element selected from the group consisting of Ni, Co and Fe, the mixture having a total mixture composition comprising by weight, 10-35% Cr, 4-10% Al, from a small but effective amount of Y, 2-6% Si, with the balance M, the Si being present solely as a diffused element into the lower temperature range melting one of the powdered alloys, the outer portion having a thickness in the range of at least about 0.05xe2x80x3.
The invention further provides an article including an environmentally resistant, abradable surface portion comprising:
an inner portion of a superalloy selected from a group consisting of Co-base, Ni-base and Fe-base superalloys and having an incipient melting temperature; and
an outer portion metallurgically bonded with the inner portion, and consolidated and liquid phase sintered from the mixture having a total mixture composition comprising by weight, 10-35% Cr, 4-10% Al, from a small but effective amount of Y, 2-6% Si, with the balance M, the Si being present solely as a diffused element into the first of the powdered alloys, and further characterized by the substantial absence of B;
the first of the powdered alloys having a solidus and a liquidus, constituting 30-50% by weight of the mixture, being characterized by the substantial absence of Y, and comprising, by weight, 8-12% Si, 1.5-4% Al, 10-30% Cr, with the balance M,
the second of the powdered alloys having a solidus and a liquidus, constituting 50-70% by weight of the mixture and having a melting range higher than the first of the powdered alloys;
the second alloy characterized by the substantial absence of Si,
the first and/or second powdered alloys characterized by the presence of Y in an amount, up to a total of about 1% by weight, which will provide the Y content in the total mixture composition, the outer portion having a thickness in the range of at least about 0.05xe2x80x3.
The invention further provides a turbine shroud as the article, and a turbine shroud in which
the outer portion is consolidated and liquid phase sintered from the mixture additionally comprising polytetrafluoroethylene in HF generating amount sufficient to strip oxides from the powdered alloy at temperatures in the melting range of the second powdered alloy.
The invention provides in a further aspect a method of coating a metal part including applying to the part surface a composition comprising a resinous binder processing aid and a mixture of a first lower melting MCrAl(Y)n powdered alloy and a second higher temperature melting MCrAl(Y)n powdered alloy, n being an integer of zero or greater, such as 1, and heating the composition and part to volatilize the binder and melt the powdered alloys in metallurgical bond forming relation with the metal part.
In this and like embodiments, the method typically further includes selecting as the resinous binder a resinous binder comprising a fluorine-containing synthetic organic polymer adapted to bind the powdered alloy composition, selecting as the fluorine-containing polymer binder polytetrafluoroethylene, volatilizing HF from the polytetrafluoroethylene binder during heating of the composition, and where the metal part is surface oxide-forming, e.g. oxides of aluminum, titanium or yttrium stripping the oxides from the powdered alloys with the HF in metallurgical bonding facilitating relation between the alloys and the part surface, adding silicon to the first powdered alloy before mixing with the second powdered alloy, diffusing silicon into the first powdered alloy in a diffusion pack at elevated temperatures, adding silicon at between about 5% and about 18% by weight to the first powdered alloy in the diffusion pack, adding silicon at about 9% by weight to the first powdered alloy in the diffusion pack, and selecting an MCrAl(Y)n powdered alloy, n being an integer of zero or greater, such as 1, in which M is Ni, Co and/or Fe as the first powdered alloy.
In a further aspect, the invention provides a method of uniformly distributing silicon through a composition of powdered MCrAl alloy, including surface diffusing silicon into a portion of the powdered alloy, and blending the silicon-diffused portion with the balance of the alloy.
In its compositional aspects the invention provides a metal part coating composition comprising an alloy powder-binding amount of a resinous binder and mixture of a first lower melting MCrAl(Y)n powdered alloy and a second higher temperature melting MCrAl(Y)n powdered alloy, n being an integer of zero or greater, the binder being volatile at the melting temperature of the alloys.
In this and like embodiments, typically, the first powdered alloy further comprises silicon in an amount lowering the melting temperature range of the powdered alloy, the silicon is surface-diffused into the first powdered alloy, the first powdered alloy further comprises from about 5% to about 18% by weight of the surface-diffused silicon, the resinous binder comprises a fluorine-containing synthetic organic polymer adapted to bind the alloy composition, the binder comprises polytetrafluoroethylene, the polytetrafluoroethylene binder comprises Teflon 6C, and, the polymer volatilizes HF at the melting temperatures of the higher melting of the powdered alloys.
In another aspect the invention provides a metal coating composition comprising an intimately interdistributed mixture of a first lower melting MCrAl(Y)n powdered alloy, a second higher temperature melting MCrAl(Y)n powdered alloy, n being an integer of zero or greater, and an alloy powder-binding amount of a resinous binder, the binder being volatile at the melting temperature of the higher temperature melting of the alloys.
In this and like embodiments of the invention, typically, the first powdered alloy further comprises silicon diffused thereinto in fusion temperature lowering amount.
In yet another aspect of the invention there is provided a preformed metal coating product comprising compressed into a predetermined form adapted to coat a metal surface an intimately interdistributed mixture of a first lower melting MCrAl(Y)n powdered alloy, a second higher temperature melting MCrAl(Y)n powdered alloy, n being an integer of zero or greater, and an alloy powder-binding amount of a resinous binder, the binder being volatile at the melting temperature of the higher temperature melting of the alloys.
In this and like embodiments, the product is formed of multiple, separable layers of generally uniform composition with each other, the product comprises from about 15% to about 55% by weight of the first powdered alloy, the balance the second powdered alloy and sufficient binder to maintain the product in its the predetermined form, the product is self-supporting and flexible in its the predetermined form, the product has a laminar structure permitting successive separation of layers of the product from the balance of the product, the layers having the composition of the balance of the product and greater flexibility than the product before separation of successive layers, the product is sufficiently flexible at a one-eighth inch thickness to bend 90xc2x0 without breaking at the fold locus, the first powdered alloy further comprises silicon in an amount effecting the first powdered alloy lower melting temperature, the first alloy powder silicon content is diffused thereinto, the first alloy powder contains from about 5% to about 18% by weight diffused silicon, the resinous binder comprises a fluorine-containing synthetic organic polymer adapted to bind the alloy mixture in its the predetermined form, the binder comprises polytetrafluoroethylene polymer, the polytetrafluoroethylene comprises Teflon 6 and is present in an amount between about 0.1% and about 1.0% by weight of the product, and the polymer volatilizes HF at the melting temperature of the higher melting of the powdered alloys.
In yet another embodiment, the invention provides a laminate comprising:
an inner portion of a superalloy selected from a group consisting of Co-base, Ni-base and Fe-base superalloys; and
an outer portion comprising a consolidated and liquid phase sinterable mixture of two powdered alloys each comprising a base of elements M, Cr and Al in which M is at least one element selected from the group consisting of Ni, Co and Fe, the mixture having a total mixture composition comprising by weight, 10-35% Cr, 4-10% Al, from a small but effective amount up to about 0.09% to 1% Y, 2-6% Si, with the balance M, the Si being an alloy constituent or a diffused element and present solely in the lower temperature range melting one of the powdered alloys, the outer portion mixture having a thickness in the range of at least about 0.05 inch; or the laminate comprising:
an inner portion of a superalloy selected from a group consisting of Co-base, Ni-base and Fe-base superalloys; and
an outer portion comprising a consolidated and liquid phase sinterable mixture of a polytetrafluoroethylene binder and two powdered alloys each comprising a base of elements M, Cr and Al in which M is at least one element selected from the group consisting of Ni, Co and Fe, the mixture having a total mixture composition comprising by weight, 10-35% Cr, 4-10% Al, from a small but effective amount up to about 0.09% to 1% Y, 2-6% Si, with the balance M, the outer portion mixture having a thickness in the range of at least about 0.05 inch.
In a further aspect the invention provides the above article substantially free of oxides such as aluminum oxides in the metallurgical bond by HF removal of the oxides during liquid phase sintering.
The invention further provides an article comprising:
an inner portion of a superalloy selected from a group consisting of Co-base, Ni-base and Fe-base superalloys; and
an outer portion metallurgically bonded with the inner portion, and consolidated and liquid phase sintered from a mixture of two powdered alloys each comprising a base of elements M, Cr and Al in which M is at least one element selected from the group consisting of Ni, Co and Fe, one of the powdered alloys containing Si as a diffused element, e.g. in the form of a turbine shroud, or in the form of a turbine blade or vane.